The manufacture of diverse paper qualities has long involved the intermixing of, e.g. mechanical pulps which have been bleached to a high degree of brightness. In present day paper manufacturing processes there is a desire to increase the use of bleached mechanical and chemimechanical pulps. The great drawback with these particular pulps, and one which has retarded their use in, e.g. paper manufacture, resides in the poor brightness stability of such pulps. The brightness of these pulps thus fades much too rapidly with time, i.e. the pulps are yellowing.
Attempts to find ways and means of retarding the tendencies of such pulps for yellowing have been undertaken for several decades. One method that has been proposed in an attempt to solve this problem on an industrial scale when producing paper which contains a bleached pulp that contains a high proportion of lignin involves coating the paper produced with an appropriate chemical, e.g. a pigment, such as titanium dioxide. This titanium dioxide coating makes it difficult for light to penetrate the paper sheet and prevents yellowing to a corresponding degree. This method of solving the problem is not particularly effective, however, and it can be said that the problem of yellowing has not been solved satisfactorily hitherto.